High resolution spectroscopy of BaCH₃(X̃²A₁): Fine and hyperfine structure analysis

The pure rotational spectrum of BaCH₃(X̃²A₁) in its ground vibrational state has been recorded using millimeter/submillimeter direct absorption techniques, the first spectroscopic information obtained for this molecule. The radical was created using Broida-type oven/d.c. discharge methods by the reaction of barium vapor and Sn(CH₃)₄. Twenty-eight rotational transitions of the main isotopomer ¹³⁸BaCH₃ were recorded, as well as five for ¹³⁶BaCH₃ and three for the ¹³⁷BaCH₃ species. Being a prolate symmetric top, K ladder structure was observed in all transitions for BaCH₃, as well as fine structure splittings which arise from the unpaired electron in the molecule. For the ¹³⁷Ba isotopomer, hyperfine interactions were also resolved, arising from the spin of the barium nucleus. The complete data set has been analyzed with a ²A Hamiltonian, and rotational, spin-rotational, and magnetic hyperfine/nuclear quadrupole parameters accurately determined. The fine and hyperfine structure constants established from this study suggest a predominantly ionic bond for BaCH₃, but with a considerable covalent component. Structural parameters for BaCH₃ derived in this work are consistent with those of other alkaline earth monomethyl species.